A major goal of the NIAID Division of AIDS (DAIDS) Clinical Research Program (CRP) and its investigators are the development of new therapies for HIV-related opportunistic infections. The AIDS Clinical Trials Group (ACTG) and the Community Program for Clinical Research on AIDS (CPCRA) have begun research programs to evaluate treatment and prophylaxis of Mycobacterium avium-intracellulare (MAI) infections in HIV-infected patients. Protocols evaluating MAI treatments will utilize quantitative blood cultures as an important response parameter which, in conjunction with clinical status, will determine patient eligibility for trials and will be used to assess the efficacy of study therapies. For a multicenter clinical trial to reach valid conclusions, it is imperative that quantitation be performed in a standardized and reproducible fashion. There is no widely available standardized methodology for quantitative culture of MAI, and there is a need for a referecne laboratory both to perform this service and to continue development of such methodology. Such a facility would also serve as a microbiologic repository, maintaining a collection of clinical isolates for future analysis. Clinical trials evaluating MAI therapy will also require in vitro susceptibility testing of isolates. The treatment of MAI is difficult, as most isolates are resistant to available antibiotics. Correlation of initial in vitro sensitivities with clinical and microbiological outcomes will be valuable, as will sequential testing for development of resistance during treatment. As is the case for quantitative cultures, there is no widely accepted standardized methodology for susceptibility testing of MAI isolates. A central reference laboratory to support the clinical trials effort is needed to appropriately compare the isolates recovered from patients at multiple institutions and to refine and adapt available methodologies to do so.